1. Field of the Invention
The present invention relates to load control devices for controlling the amount of power delivered to an electrical load from a power source. More specifically, the present invention relates to a touch dimmer having a touch sensitive device.
2. Description of the Related Art
A conventional two-wire dimmer has two terminals: a “hot” terminal for connection to an alternating-current (AC) power supply and a “dimmed hot” terminal for connection to a lighting load. Standard dimmers use one or more semiconductor switches, such as triacs or field effect transistors (FETs), to control the current delivered to the lighting load and thus to control the intensity of the light. The semiconductor switches are typically coupled between the hot and dimmed hot terminals of the dimmer.
Smart wall-mounted dimmers include a user interface typically having a plurality of buttons for receiving inputs from a user and a plurality of status indicators for providing feedback to the user. These smart dimmers typically include a microcontroller or other processing device for providing an advanced set of control features and feedback options to the end user. An example of a smart dimmer is described in greater detail in commonly assigned U.S. Pat. No. 5,248,919, issued on Sep. 28, 1993, entitled LIGHTING CONTROL DEVICE, which is herein incorporated by reference in its entirety.
FIG. 1 is a front view of a user interface of a prior art smart dimmer switch 10 for controlling the amount of power delivered from a source of AC power to a lighting load. As shown, the dimmer switch 10 includes a faceplate 12, a bezel 14, an intensity selection actuator 16 for selecting a desired level of light intensity of a lighting load (not shown) controlled by the dimmer switch 10, and a control switch actuator 18. Actuation of the upper portion 16A of the intensity selection actuator 16 increases or raises the light intensity of the lighting load, while actuation of the lower portion 16B of the intensity selection actuator 16 decreases or lowers the light intensity. The intensity selection actuator 16 may control a rocker switch, two separate push switches, or the like. The control switch actuator 18 may control a push switch or any other suitable type of actuator and typically provides tactile and auditory feedback to a user when pressed.
The smart dimmer 10 also includes an intensity level indicator in the form of a plurality of light sources 20, such as light-emitting diodes (LEDs). Light sources 20 may be arranged in an array (such as a linear array as shown) representative of a range of light intensity levels of the lighting load being controlled. The intensity level of the lighting load may range from a minimum intensity level, which is preferably the lowest visible intensity, but which may be zero, or “full off,” to a maximum intensity level, which is typically “full on.” Light intensity level is typically expressed as a percentage of full intensity. Thus, when the lighting load is on, light intensity level may range from 1% to 100%.
By illuminating a selected one of the light sources 20 depending upon light intensity level, the position of the illuminated light source within the array provides a visual indication of the light intensity relative to the range when the lamp or lamps being controlled are on. For example, seven LEDs are illustrated in FIG. 1. Illuminating the uppermost LED in the array will give an indication that the light intensity level is at or near maximum. Illuminating the center LED will give an indication that the light intensity level is at about the midpoint of the range. In addition, when the lamp or lamps being controlled are off, all of the light sources 18 are illuminated at a low level of illumination, while the LED representative of the present intensity level in the on state is illuminated at a higher illumination level. This enables the light source array to be more readily perceived by the eye in a darkened environment, which assists a user in locating the switch in a dark room, for example, in order to actuate the switch to control the lights in the room, and provides sufficient contrast between the level-indicating LED and the remaining LEDs to enable a user to perceive the relative intensity level at a glance.
Touch dimmers (or “zip” dimmers) are known in the art. A touch dimmer generally includes a touch-operated input device, such as a resistive or a capacitive touch pad. The touch-operated device responds to the force and position of a point actuation on the surface of the device and in turn controls the semiconductor switches of the dimmer. An example of a touch dimmer is described in greater detail in commonly-assigned U.S. Pat. No. 5,196,782, issued Mar. 23, 1993, entitled TOUCH-OPERATED POWER CONTROL, the entire disclosure of which is hereby incorporated by reference.
FIG. 2 is a cross-sectional view of a prior art touch-operated device 30, specifically, a membrane voltage divider. A conductive element 32 and a resistive element 34 are co-extensively supported in close proximity by a spacing frame 36. An input voltage, VIN, is applied across the resistive element 34 to provide a voltage gradient across its surface. When pressure is applied at a point 38 along the conductive element 32 (by a finger or the like), the conductive element flexes downward and electrically contacts a corresponding point along the surface of the resistive element 34, providing an output voltage, VOUT, whose value is between the input voltage VIN and ground. When pressure is released, the conductive element 32 recovers its original shape and becomes electrically isolated from the resistive element 34. The touch-operated device 30 is characterized by a contact resistance RCONTACT between the conductive element 32 and the resistive element 34. The contact resistance RCONTACT is dependent upon the force of the actuation of the touch-operated device 30 and is typically substantially small for a normal actuation force.
FIG. 3 is a perspective view of a user interface of a prior art touch dimmer 40. The dimmer 40 comprises a touch-operated device 30, which is located directly behind a faceplate 42. The faceplate 42 includes a flexible area 44 located directly above the conductive element 32 of the touch-operated device 30 to permit a user to actuate the touch-operated device through the faceplate 42. A conventional phase-control dimming circuit is located within an enclosure 46 and controls the power from a source to a load in accordance with pressure applied to a selectable point on flexible area 44. The faceplate 42 may include optional markings 48, 50, 52 to indicate, respectively, the location of flexible area 44, the lowest achievable intensity level of the load, and location of a “power off” control. An optional LED array 54 provides a visual indication of intensity level of the load. When the load is a light source, there is preferably a linear relationship between the number of illuminated LEDs and the corresponding perceived light level. The flexible area 44 may optionally include a light transmissive area through which LED array 54 is visible.
It is desirable to provide a touch dimmer that is responsive to only the position of an actuation on the operational area, e.g., the flexible area 44 of the touch dimmer 40. However, most prior art touch dimmers are responsive to both the force and the position of a point actuation. For example, when a user lightly presses the touch-operated device 30, i.e., with a low actuation force, the contact resistance RCONTACT is substantially larger than during a normal actuation. Accordingly, the output of the touch-operated device 30 is not representative of the position of the actuation and the dimmer 40 may control the lighting load to an undesired intensity level. Therefore, there is a need for a touch dimmer having an operational area that is not responsive to light touches and is responsive to only the position of a point actuation.